Communication networks based on optical fibers for transferring data between terminals are attractive because of the high bandwidth of the optical fibers. Data is transmitted on these networks by modulating a light source, usually a laser. A plurality of users may share a single fiber either via time or wave division multiplexing (WDM). Wave division multiplexing is typically implemented by assigning a different wavelength to each user or channel.
A limited number of stations may be accommodated on any given fiber ring. Accordingly, a coupling device is used to selectively couple data from a first fiber to a second fiber. The data to be coupled is typically isolated to a sub-set of the channels on the first fiber. In some cases, it is advantageous to simultaneously remove the transferred channel from the first fiber after the channel is connected to the second fiber. Such coupling devices include optical circulators.
In a telecommunications network each subscriber communicates with a central office over a fiber that is arranged in a ring with the subscriber and central office stations disposed along the ring. If the fiber is broken, communication between one or more of the users and the central office will be interrupted. In principle, these users can still communicate with the central office by sending messages along the uninterrupted portion of the loop. However, this requires that the direction of propagation along the fiber be reversed over a portion of the fiber.
Unfortunately, the fiber ring typically includes components that are unidirectional in nature such as the optical circulators used to couple the fibers discussed above. To reverse the direction of propagation in response to a fiber break, duplicate optical circulators configured to propagate signals in the opposite direction are included in the network. These components are inserted into the fiber in place of the corresponding components by utilizing bypass switches. Such bypass arrangements substantially increase the cost and complexity of the optical network, and hence, it would be advantageous to avoid these bypass arrangements.
Broadly, it is the object of the present invention to provide an improved optical circulator.
It is another object of the present invention to provide an optical circulator whose direction of light transmission can be reversed by applying a control signal to the optical circulator without the need to utilize bypass switches and additional optical circulators.
It is yet another object of the present invention to provide an optical coupling arrangement in which the direction of propagation of the coupled signals may be switched without the need to utilize bypass switches and other circulators.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.